Autonomic Nervous System Drugs — MCQs

Autonomic Nervous System Drugs Indian Medical PG Practice Questions and MCQs

Question 251: Succinylcholine acts to block neuromuscular transmission by:

A. Inhibiting cholinesterase
B. Inhibiting the central nervous system
C. Depolarizing the motor end plate of skeletal muscle (Correct Answer)
D. Blocking the release of acetylcholine at the end plate

Explanation: **Mechanism of Action** Succinylcholine (Suxamethonium) is the only **depolarizing neuromuscular blocker** used clinically. It acts as a nicotinic acetylcholine receptor (nAChR) agonist. Because it is not metabolized by acetylcholinesterase at the synapse, it persists longer than acetylcholine, causing persistent stimulation of the motor end plate. This leads to initial muscle fasciculations followed by a flaccid paralysis because the sodium channels remain in an inactivated state, preventing further action potentials (Phase I block). **Analysis of Options** * **Option A (Inhibiting cholinesterase):** This is the mechanism of drugs like Neostigmine or Organophosphates. Inhibiting cholinesterase actually *increases* acetylcholine levels, which would worsen a Phase I block caused by succinylcholine. * **Option B (Inhibiting the CNS):** Succinylcholine is a highly polar quaternary ammonium compound; it does not cross the blood-brain barrier and has no central effect. * **Option D (Blocking ACh release):** This is the mechanism of **Botulinum toxin** or aminoglycosides (at high doses). Succinylcholine acts postsynaptically, not presynaptically. **NEET-PG High-Yield Pearls** * **Metabolism:** Rapidly hydrolyzed by **Pseudocholinesterase** (Butyrylcholinesterase) in the plasma. * **Genetic Variation:** Patients with atypical pseudocholinesterase experience prolonged apnea (diagnosed via **Dibucaine Number**). * **Adverse Effects:** Hyperkalemia (dangerous in burn/trauma patients), Malignant Hyperthermia (treated with **Dantrolene**), and postoperative myalgia. * **Phase II Block:** With prolonged infusion, the membrane repolarizes but becomes desensitized to ACh, resembling a non-depolarizing block.

Question 252: The action of botulinum toxin is through which mechanism?

A. Direct action on acetylcholine esterase
B. Prevention of choline uptake
C. Direct action on muscarinic receptors
D. Prevention of acetylcholine release (Correct Answer)

Explanation: **Explanation:** **Mechanism of Action:** Botulinum toxin (produced by *Clostridium botulinum*) acts by inhibiting the release of **Acetylcholine (ACh)** from the presynaptic nerve terminals at the neuromuscular junction and autonomic ganglia. It achieves this by enzymatically cleaving **SNARE proteins** (specifically Synaptobrevin, SNAP-25, and Syntaxin). These proteins are essential for the docking and fusion of ACh-containing vesicles with the presynaptic membrane; without them, exocytosis cannot occur, leading to flaccid paralysis. **Analysis of Incorrect Options:** * **Option A:** Drugs that act on Acetylcholine esterase are known as anticholinesterases (e.g., Neostigmine, Organophosphates). They inhibit the breakdown of ACh rather than preventing its release. * **Option B:** Prevention of choline uptake is the mechanism of **Hemicholinium**, which blocks the CHT1 transporter, leading to depletion of ACh synthesis. * **Option C:** Direct action on muscarinic receptors is characteristic of cholinergic agonists (e.g., Pilocarpine, Bethanechol) or antagonists (e.g., Atropine). **NEET-PG High-Yield Pearls:** * **Clinical Uses:** Used for focal dystonias (Blepharospasm, Spasmodic torticollis), Achalasia cardia, Hyperhidrosis, and cosmetic reduction of wrinkles. * **Black Box Warning:** Potential for systemic spread of toxin causing botulism-like symptoms (respiratory paralysis). * **Contrast with Tetanus Toxin:** While both cleave SNARE proteins, Tetanus toxin undergoes retrograde axonal transport to the spinal cord and inhibits **GABA/Glycine** release (inhibitory neurotransmitters), leading to spastic paralysis (Lockjaw).

Question 253: Which alpha-blocker is useful in Benign Prostatic Hyperplasia (BPH)?

A. Phentolamine
B. Prazosin (Correct Answer)
C. Tolazoline
D. Phenoxybenzamine

Explanation: ### Explanation **Correct Option: B. Prazosin** The therapeutic goal in Benign Prostatic Hyperplasia (BPH) is to relax the smooth muscles of the prostatic urethra and bladder neck to improve urine flow. This is mediated by **$\alpha_1$-adrenergic receptors**. Prazosin is a **selective $\alpha_1$-blocker** [1]. By blocking these receptors, it decreases the dynamic component of urinary obstruction without significantly affecting $\alpha_2$ receptors (which would otherwise cause tachycardia) [4]. While newer uroselective agents like Tamsulosin ($\alpha_{1A}$ specific) are now preferred [3], Prazosin remains a classic prototype used for this indication. **Analysis of Incorrect Options:** * **A. Phentolamine:** This is a **non-selective, competitive $\alpha$-blocker**. Because it blocks $\alpha_2$ receptors (leading to increased norepinephrine release), it causes significant reflex tachycardia, making it unsuitable for chronic conditions like BPH. It is primarily used in hypertensive crises (e.g., Pheochromocytoma) [5]. * **C. Tolazoline:** Similar to Phentolamine, it is a **non-selective $\alpha$-blocker** with histamine-like effects. It was historically used for persistent pulmonary hypertension in neonates but is not used for BPH. * **D. Phenoxybenzamine:** This is a **non-selective, irreversible $\alpha$-blocker**. Its long duration of action and significant side effect profile (orthostatic hypotension and reflex tachycardia) make it inappropriate for BPH management. It is the drug of choice for the preoperative management of Pheochromocytoma [2]. **NEET-PG High-Yield Pearls:** * **Uroselectivity:** Tamsulosin and Silodosin are selective for **$\alpha_{1A}$ receptors** (found in the prostate), causing less hypotension than Prazosin [3]. * **First-Dose Phenomenon:** Prazosin can cause sudden severe orthostatic hypotension; patients should take the first dose at bedtime [1]. * **Dual Benefit:** Prazosin is an excellent choice for a patient suffering from both **Hypertension and BPH**. * **Side Effect:** Watch for "Floppy Iris Syndrome" during cataract surgery in patients taking $\alpha_1$-blockers.

Question 254: Which local anesthetic raises blood pressure instead of tending to cause a fall?

A. Cocaine (Correct Answer)
B. Dibucaine
C. Lignocaine
D. Procaine

Explanation: **Explanation:** The correct answer is **Cocaine**. **1. Why Cocaine is correct:** Most local anesthetics (LAs) are vasodilators and can cause hypotension by relaxing vascular smooth muscle and depressing myocardial contractility. Cocaine is the unique exception. It acts as a **sympathomimetic** by blocking the reuptake of norepinephrine (NE) at sympathetic nerve endings (NET inhibition). This leads to an accumulation of NE in the synaptic cleft, causing potent vasoconstriction, tachycardia, and a significant **increase in blood pressure**. **2. Why the other options are incorrect:** * **Lignocaine (Lidocaine):** The most widely used LA; it causes vasodilation and can lead to hypotension if absorbed systemically in large doses. * **Procaine:** An ester-linked LA with significant vasodilatory properties and a short duration of action. * **Dibucaine:** A potent, long-acting amide LA (often used in spinal anesthesia) that, like others, typically causes a fall in BP due to sympathetic blockade. **3. NEET-PG High-Yield Pearls:** * **Vasoconstriction:** Cocaine is the only naturally occurring local anesthetic and the only one that causes vasoconstriction. * **Clinical Use:** Due to its vasoconstrictive property, it is used topically in ENT surgeries to reduce bleeding (e.g., nasal packing). * **Cardiotoxicity:** While most LAs cause bradycardia, cocaine causes tachycardia and arrhythmias. * **Adrenaline Interaction:** Never co-administer adrenaline with cocaine, as it can lead to a hypertensive crisis or fatal arrhythmias. * **Metabolism:** Unlike other esters (metabolized by plasma pseudocholinesterase), cocaine is primarily metabolized in the liver.

Question 255: Which adrenergic beta-receptor subtype is primarily responsible for lipolysis in fat cells?

A. Alpha-1
B. Alpha-2
C. Beta-1
D. Beta-3 (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Beta-3)** The **Beta-3 ($\beta_3$) receptor** is the primary adrenergic receptor subtype located in adipose tissue (both white and brown fat) [1], [3]. Activation of these receptors by norepinephrine or selective agonists stimulates the enzyme **adenylyl cyclase**, increasing intracellular cAMP. This activates **hormone-sensitive lipase**, which catalyzes the breakdown of triglycerides into free fatty acids and glycerol—a process known as **lipolysis** [1], [3]. In brown adipose tissue, $\beta_3$ stimulation also mediates **thermogenesis** [3]. **Analysis of Incorrect Options:** * **Alpha-1 ($\alpha_1$):** These are primarily Gq-coupled receptors found in vascular smooth muscle, causing vasoconstriction and pupillary dilation (mydriasis). They do not play a significant role in lipid metabolism. * **Alpha-2 ($\alpha_2$):** These are inhibitory (Gi-coupled) receptors. While they are present on fat cells, their activation actually **inhibits lipolysis** by decreasing cAMP levels [2]. * **Beta-1 ($\beta_1$):** These are predominantly found in the **heart** (increasing heart rate and contractility) and the **juxtaglomerular apparatus** of the kidney (increasing renin release) [1]. While they have a minor metabolic role, $\beta_3$ is the specific and primary mediator for lipolysis [3]. **High-Yield NEET-PG Pearls:** * **Mirabegron:** A selective $\beta_3$ agonist used clinically for the treatment of **overactive bladder** (it relaxes the detrusor muscle). * **Location Summary:** * $\beta_1$: Heart (1 heart) * $\beta_2$: Lungs/Bronchi (2 lungs) and Uterus (Tocolysis) * $\beta_3$: Adipose tissue (Lipolysis) and Detrusor muscle (Relaxation) [3] * **Metabolic effects of Catecholamines:** Generally, $\beta$-receptors increase blood glucose (glycogenolysis/gluconeogenesis) and free fatty acids (lipolysis) to provide energy during "fight or flight" responses.

Question 256: Which of the following is the antidote for beta-blocker poisoning?

A. Calcium gluconate
B. Glucagon (Correct Answer)
C. Sodium phosphate
D. Chloride salts

Explanation: **Explanation:** **Why Glucagon is the Correct Answer:** Glucagon is considered the specific antidote for beta-blocker overdose. Beta-blockers cause bradycardia and hypotension by blocking $\beta_1$ receptors, which normally use the Gs-protein pathway to activate adenyl cyclase and increase intracellular cAMP. Glucagon bypasses these blocked $\beta$-receptors by binding to its own specific **glucagon receptors** on the myocardium. This binding independently activates adenyl cyclase, leading to an increase in **cAMP**, which exerts positive inotropic (contractility) and chronotropic (heart rate) effects. Essentially, it restores cardiac function through an alternative pathway. **Analysis of Incorrect Options:** * **A. Calcium gluconate:** While intravenous calcium can be used to improve contractility, it is primarily the first-line antidote for **Calcium Channel Blocker (CCB)** poisoning, not beta-blockers. * **C. Sodium phosphate:** This has no role in treating beta-blocker toxicity; it is typically used for electrolyte replacement in hypophosphatemia. * **D. Chloride salts:** These are not used as antidotes here. Sodium chloride (Normal Saline) is used for volume resuscitation, but it does not reverse the pharmacological blockade. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Glucagon:** Increases cAMP via non-adrenergic pathways. * **Other treatments for Beta-blocker toxicity:** High-dose Insulin-Euglycemia Therapy (HIET) is another critical management strategy if glucagon fails. * **Atropine:** Often the first drug tried for bradycardia, but frequently ineffective in severe beta-blocker overdose. * **Membrane Stabilizing Activity (MSA):** Beta-blockers like Propranolol can cause QRS widening; in such cases, **Sodium Bicarbonate** is indicated.

Question 257: What is the effect of atropine on the eye?

A. Active mydriasis
B. Passive mydriasis (Correct Answer)
C. Miosis
D. Pinpoint pupil

Explanation: **Explanation:** The correct answer is **Passive Mydriasis**. To understand this, we must look at the autonomic control of the iris muscles: 1. **Mechanism of Action:** Atropine is a competitive muscarinic antagonist. In the eye, it blocks the **M3 receptors** on the **Sphincter Pupillae** (circular muscle). Normally, Parasympathetic stimulation causes this muscle to contract, leading to miosis. By blocking this action, atropine causes the muscle to relax. The pupil dilates because the opposing muscle (Dilator Pupillae) is now unopposed. This is termed **"Passive"** because the dilation results from relaxation of the constrictor, not active contraction of the dilator. **Analysis of Options:** * **Active Mydriasis (Option A):** This occurs when **Alpha-1 agonists** (like Phenylephrine) directly stimulate the **Dilator Pupillae** (radial muscle) to contract. * **Miosis (Option C):** This is pupillary constriction caused by parasympathomimetics (like Pilocarpine) or opioid toxicity. * **Pinpoint Pupil (Option D):** This is extreme miosis, typically seen in Organophosphate poisoning or Pontine hemorrhage. **High-Yield NEET-PG Pearls:** * **Cycloplegia:** Atropine also blocks M3 receptors on the **Ciliary Muscle**, leading to loss of accommodation (paralysis of near vision). * **Duration:** Atropine has the longest duration of action among mydriatics (7–10 days). For refraction testing in adults, shorter-acting agents like **Tropicamide** are preferred. * **Contraindication:** Atropine is strictly contraindicated in **Narrow-Angle Glaucoma**, as mydriasis can further obstruct the aqueous outflow facility.

Question 258: Which of the following drugs is a synthetic choline ester?

A. Nicotine
B. Pilocarpine
C. Arecoline
D. Carbachol (Correct Answer)

Explanation: **Explanation:** The correct answer is **Carbachol**. **1. Why Carbachol is correct:** Cholinergic agonists (Parasympathomimetics) are classified into two main groups: **Choline Esters** and **Alkaloids**. Choline esters are synthetic or semi-synthetic analogs of Acetylcholine. Carbachol is a synthetic choline ester that possesses both muscarinic and nicotinic activity. Unlike Acetylcholine, it is resistant to hydrolysis by acetylcholinesterase (AChE) due to its carbamate group, resulting in a longer duration of action. Other examples in this category include Bethanechol and Methacholine. **2. Why the other options are incorrect:** * **Nicotine (A):** This is a naturally occurring **alkaloid** derived from the tobacco plant (*Nicotiana tabacum*). It acts primarily on nicotinic receptors. * **Pilocarpine (B):** This is a natural **alkaloid** obtained from the leaves of *Pilocarpus microphyllus*. It is a tertiary amine and is primarily used in the treatment of glaucoma and xerostomia. * **Arecoline (C):** This is a natural **alkaloid** found in the betel nut (*Areca catechu*). Like pilocarpine, it is not a choline ester. **3. NEET-PG High-Yield Pearls:** * **Bethanechol:** The only choline ester that is purely muscarinic (M-selective) and has negligible nicotinic action. It is used for post-operative urinary retention. * **Methacholine:** Used in the "Methacholine Challenge Test" to diagnose bronchial hyperreactivity (Asthma). * **Resistance to AChE:** Choline esters with a carbamate group (Carbachol, Bethanechol) are resistant to AChE, whereas those with a methyl group (Methacholine) are only partially resistant. * **Pilocarpine** is the drug of choice for emergency lowering of intraocular pressure in acute angle-closure glaucoma.

Question 259: Which one of the following sites is characterized by adrenergic neurohumoral transmission?

A. Parasympathetic preganglionic fibers
B. Sympathetic postganglionic fibers
C. Sympathetic fibers in the adrenal medulla (Correct Answer)
D. Synaptic fibers in the eccrine gland

Explanation: ### Explanation The core concept of neurohumoral transmission in the Autonomic Nervous System (ANS) depends on the specific neurotransmitter released at the nerve terminal. **Why Option C is Correct:** The **adrenal medulla** is essentially a modified sympathetic ganglion. It is innervated by preganglionic sympathetic fibers; however, the chromaffin cells within the medulla act as postganglionic neurons. These cells synthesize, store, and release **Catecholamines** (80% Epinephrine/Adrenaline and 20% Norepinephrine) directly into the bloodstream. Therefore, the transmission within the adrenal medulla is fundamentally **adrenergic** in nature. **Analysis of Incorrect Options:** * **Option A (Parasympathetic preganglionic fibers):** All preganglionic fibers (both sympathetic and parasympathetic) are **cholinergic**, meaning they release Acetylcholine (ACh) to act on nicotinic receptors ($N_m$). * **Option B (Sympathetic postganglionic fibers):** While *most* sympathetic postganglionic fibers are adrenergic (releasing Norepinephrine), the question asks for a specific site. Option C is a more definitive representation of adrenergic transmission involving the release of Adrenaline. * **Option D (Sympathetic fibers in eccrine glands):** This is a classic "exception" in pharmacology. Although these are sympathetic postganglionic fibers, they are **cholinergic**. They release ACh to stimulate sweat production via muscarinic ($M_3$) receptors. **High-Yield NEET-PG Pearls:** * **The Exceptions:** All sympathetic postganglionic fibers are adrenergic **EXCEPT** those to sweat glands (eccrine) and some blood vessels in skeletal muscle (vasodilator fibers), which are cholinergic. * **Neurotransmitter Synthesis:** The rate-limiting step in the synthesis of catecholamines (Adrenaline/Noradrenaline) is the conversion of Tyrosine to DOPA by the enzyme **Tyrosine Hydroxylase**. * **Adrenal Medulla:** It is the only site that can convert Norepinephrine to Epinephrine due to the presence of the enzyme **PNMT** (Phenylethanolamine N-methyltransferase), which is induced by cortisol.

Question 260: Alpha 2 agonists cause all of the following except?

A. Analgesia
B. Hyperalgesia (Correct Answer)
C. Sedation
D. Anxiolysis

Explanation: **Explanation:** Alpha-2 ($\alpha_2$) agonists (such as **Clonidine** and **Dexmedetomidine**) act primarily by stimulating presynaptic $\alpha_2$ receptors in the central nervous system. This leads to a decrease in the release of norepinephrine, resulting in a reduction of sympathetic outflow. **Why Hyperalgesia is the Correct Answer:** Hyperalgesia refers to an increased sensitivity to pain. $\alpha_2$ agonists are known for their **antinociceptive** properties, meaning they inhibit pain transmission. Therefore, they cause **analgesia**, not hyperalgesia. Hyperalgesia is often a side effect of chronic opioid use (Opioid-Induced Hyperalgesia) or certain inflammatory states, but it is physiologically opposite to the effects of $\alpha_2$ stimulation. **Analysis of Incorrect Options:** * **Analgesia:** $\alpha_2$ receptors are located in the dorsal horn of the spinal cord. Their activation inhibits the release of substance P and glutamate, effectively blocking pain signals. * **Sedation:** Stimulation of $\alpha_2$ receptors in the **locus coeruleus** (the brain's primary noradrenergic nucleus) reduces wakefulness, leading to a "natural-like" non-REM sleep state. * **Anxiolysis:** By reducing central sympathetic activity and norepinephrine levels, these drugs effectively lower states of arousal and anxiety. **High-Yield Clinical Pearls for NEET-PG:** * **Dexmedetomidine:** Highly selective $\alpha_2$ agonist used for ICU sedation; it is unique because it causes sedation without significant respiratory depression. * **Clonidine:** Used in hypertensive emergencies, opioid withdrawal, and ADHD. * **Side Effects:** The most common side effects are **bradycardia** and **hypotension** (due to decreased sympathetic tone) and xerostomia (dry mouth). * **Apraclonidine/Brimonidine:** Topical $\alpha_2$ agonists used to reduce intraocular pressure in glaucoma by decreasing aqueous humor production.

Practice by Chapter

Cholinergic Agonists

Practice Questions

Cholinergic Antagonists

Practice Questions

Adrenergic Agonists

Practice Questions

Adrenergic Antagonists

Practice Questions

Ganglionic Agents

Practice Questions

Neuromuscular Blocking Agents

Practice Questions

Autonomic Drugs in Ophthalmology

Practice Questions

Autonomic Drugs in Cardiovascular Disease

Practice Questions

Autonomic Drugs in Respiratory Disease

Practice Questions

Autonomic Drugs in Urological Disorders

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free